Next, we put the measured amount of egg whites, still in the 1-cup measuring cup, into a large bowl of warm water. We want the egg whites to be surrounded by the warm water, but for the water in the bowl to not get into the measuring cup. This will gently bring the egg whites up to room temperature. We set it aside for 10 minutes.
Meanwhile, we get the rest of the ingredients ready.
When the egg whites are ready, we add them to the bowl of our stand-up mixer. We also add the cream of tartar and salt. With the whisk attachment, we start on speed 2 and then gradually increase to speed 6 over the course of a minute. After 2 more minutes—when the mixture is foamy—we pour the sugar, cornstarch, vinegar and vanilla into the bowl in a stream. We increase the mixer to speed 8 and then speed 10 and continue to whip for another 4 minutes. We scrape down the sides, and check the consistency. We give it another minute or so until the mixture is thick, white, glossy and holds stiff peaks.
While the mixture is whipping, we draw a 9 inch circle (using a "Blues Clues party plate!) onto a piece of parchment paper and then place it pencil side down on our cookie sheet.
When the egg whites have reached stiff peaks, we spread them into the circle, using a spatula to smooth the sides and top. We also try to have the edges slightly higher than the center of the meringue since we will add toppings later.
The cookie sheet is placed in the center of the oven for 10 minutes. Then, the heat is reduced to 300° F and we continue to bake for another 50 minutes. We want the surface to be lightly browned. The oven is then turned off, and the door left ajar, for half an hour. This allows for a gradual cooling.
When it is time to serve, we cut a piece with a serrated knife and top with whipped cream and berries. The crisp exterior cracks and falls a bit onto the marshmallowy interior, but it all comes together with the cream and berries!